1. Field of the Invention
The invention pertains to an image forming apparatus using an electrophotographic process and more particularly to an image forming apparatus for exposing an image carrying member by an LED (light emitting diode) array.
2. Description Relative to the Prior Art
Apparatus for selectively removing electric charge on an image-carrying member using LEDs in electrophotographic copiers or printers are well known.
In LED (light-emitting diode) electrophotographic copiers or printers, several thousand LEDs are typically arranged in one or more rows for recording on a suitable photoconductive web or drum. Driver circuitry are provided for selectively activating the LEDs to emit light to record in accordance with electronic data signals. In grey level LED printers such as disclosed in PCT Publication WO 91/10311 and U.S. application Ser. No. 07/498,512, now U.S. Pat. No. 5,200,765 (the contents of both of which are incorporated herein by this reference), the data signals may be multibit digital signals for determining an exposure duration for recording each pixel. As also noted in the aforementioned publication and application, the LEDs are known to be non-uniform light emitters relative to each other and correction is therefore desirable to overcome image degradation due to non-uniformities between the LEDs. One form of correction is adjustment of pulsewidth duration for recording each pixel so that any two LEDs on the printhead, when each is enabled to record a pixel of a desired density, will provide approximately identical densities, even though the light output (intensity) from each is very different. While the known correction methods may work well with printing of grey level halftone, it is not sufficient to handle more stringent requirements of printing continuous-tone images.
In correcting for non-uniformities a particular LED will have assigned thereto a number, N, of exposure times for exposing a corresponding number, N, of pixel sizes or density. As there are thousands of LEDs but only 255 available exposure times for say an eight-bits per pixel system, any correction thus represents for most LEDs an approximation. Thus, some LEDs will be provided with better correction than others. In recording of continuous tone images artifacts resulting from inadequate correction may be visible in the images and appear as thin lines in the in-track direction of recording. In addition to errors in LED non-uniformity correction, there are similar artifacts caused by improper placement of LEDs during assembly of the LEDs on the printhead. In assembly of the printheads, arrays of LED chips are positioned in a row. A row of LEDs on any one chip array will be uniformly spaced (pitched) apart at say 400 LEDs to the inch due to the accuracy of the manufacturing process of such chips. However, LEDs on the ends of adjacent chip arrays may not be positioned so as to be also spaced or pitched with this same spacing. It is desirable and therefore an object of the invention that the above-mentioned problems can be reduced.